FIG. 1 is a system block diagram of a TFT-LCD display 10, which comprises a LCD panel 11, a source driver (or data driver) 13, a gate driver (or scan driver) 12, a timing control circuit 14, and a backlight module 15. The light source of LCD panel 11 is provided by the backlight module 15 and LCD panel 11 is driven by the source driver 13 and gate driver 12 which control images displayed. The timing control circuit 14 mainly produces timing control signals in order to control the action of source driver 13 and gate driver 12. In addition, since several sets of power supply are needed by the internal circuits, DC-DC converter can be used to produce several sets of power supply to be provided for other circuits.
In FIG. 2 is an equivalent circuit of TFT-LCD panel. As shown in FIG. 2, each sub-pixel on TFT-LCD panel 11 is mainly composed of TFT 16, liquid crystal 161, and storage capacitor (CS) 162. TFT 16 functions as a switch that switches open in order from top to bottom when the gate driver 12 scans each scan line in order, as shown in FIG. 3; when a whole row of TFTs 16 switch open, the data voltage is written by the source driver 13. The CS 162 and the liquid crystal 161 are in parallel in order to increase capacitance for maintaining the voltage of data. Therefore, the gate driver 12 is mainly used to drive the drive circuit of the gate array of LCD panel 11.
In a high-resolution TFT-LCD display, for instance, a basic display unit, or a pixel, needs three points to display three primary colors of RGB. For example, when a 3000*2400 resolution TFT-LCD display scans, waveform sent by gate driver 12 switches open TFT 16 on each line in order, and the whole array of source driver 13 then charges the whole line of display points until the voltage needed by each point is achieved to display different gray level. When the charging of one line is finished, the gate driver 12 of this line switches off the voltage, and the gate driver 12 of the next line switches on the voltage and the same row of source driver 13 charges the next line of display points. This process proceeds from one line to the next in order. When the last line of display points are charged, the charging of the first line is restarted and thus achieves the effect of displaying. The main function of gate driver 12 is thus to charge LCD panel 11 to the highest voltage or to discharge to the lowest voltage.
Since the gate driver 12 has to drive all the gates of TFT 16 on each row of TFT-LCD panel 11, thus the TFT-LCD panel 11 is itself a big load. And since gates of TFT 16 on LCD panel 11 are driven by high voltage, which means that high voltage is used to drive gates of TFT 16. The structure of a basic gate driver, as shown in FIG. 4, is composed of a shift register 120, a logic control circuit 121, a level shifter 122, and an output buffer 124. When the data to be displayed is output by the controller (not shown in the diagram), the shift register 120 reads sequentially the data to be displayed to decide the order and arrangement to drive the data. The arranged driving data is sent to the logic control circuit 121 and sent serially to the level shifter 122. Each TFT 16 on LCD panel 11 is then driven at high speed and high voltage by the level-shifted driving data through the output buffer 124. Moreover, since the whole operation of the gate driver circuit is driven by digital circuit, the shift register 120 is thus composed of a plurality of D Flip-Flops; and since the main focus of the output point is high driving power at high speed, therefore the output buffer 124 is composed of a plurality of inverters.
In addition, in order to solve the problem of image-retention effect of TFT-LCD, the technique of XAO function (power off control) is mostly used at present. XAO function means that XAO is set to low level when the display is turned off. For example, the logic low level is set to 0˜3.3 v, and thus all outputs of the gate driver will be shifted to high level at the same time and all TFT 16 will be turned on. The charge stored on the CS 162 can thus be discharged and the image-retention effect can be eliminated. However, the common method of using XAO function is to send XAO signal into logic control circuit 121 and to convert low level to high level output through level shifter 122. After the display is turned off, much charge on the capacitor will be discharged since the voltage of power supply is maintained only by the capacitor and all TFT at low level will function at the same time. Therefore, when the pulse of XAO reaches, the gate voltage of all TFT 16 are all shifted to Vgh, and thus a large current is produced at the moment in which the gate of TFT 16 on gate driver circuit is activated. This large current may cause the trace on gate driver circuit to burn. Furthermore, VDD voltage will also decrease rapidly and thus causes the conversion of the level shifter 122 to fail and the XAO function to lose efficacy.